268 GEOLOGY OF EASTERN WISCONSIN. 



LOWER MAGNESIAN LIMESTONE. 



Eeposing upon the upper face of the Potsdam sandstone lies a 

 group of silicious dolomitic beds, to which the term Lower Magne- 

 sian limestone was applied by Dr. Owen, to distinguish them from 

 the Galena and J^iagara dolomites, which constituted his Upper Mag- 

 nesian limestone. The former name has very properly been retained, 

 while the latter, for good reasons, has been discarded. The term Cal- 

 ciferous sandrock, applied to the near equivalent of the Lower Magne- 

 sian limestone at the east, is not admissible in this state, from the 

 lithological nature of the rock. 



General Character. It is, as the name implies, a magnesian lime- 

 stone or dolomite, sufficiently pure to burn to a serviceable quicklime 

 in its lower, middle and upper portions, though not in each layer of 

 these subdivisions. The chief impurities are quartz, clay, iron and 

 greensand. 



The dolomite occurs in the earthy, the granular crystalline, and the 

 cryptocrystalline forms. The second is the more pi-evalent form. 

 Even when the rock has a decided earthy aspect and fracture, exami- 

 nation with a lens often shows a large element of crystalline grains, 

 and on the other hand, in most of the cases where the crystals seem 

 to have completely blended with each other, concealing themselves in 

 the common mass, the crystalline facets are apt, upon close examina- 

 tion, to be more or less discernible, so that, except where the rock is 

 silicious, the cryptocrystalline form is not common. In some por- 

 tions the mass of the rock is formed almost exclusively of small crys- 

 tals of dolomite, rather loosely aggregated, leaving minute, angular 

 spaces between the crystalline grains, forming a very pronounced 

 granular rock. There are occasional evidences of what is probably 

 the common fact, that this crystalline structure was assumed after 

 the deposition of the beds, and it may have been synchronous with 

 their dolomization. The silicious material is present in four princi- 

 pal forms: that of dissemination through the mass of the limestone, 

 of concentration in nodules of chert or flint, of aggregations of quartz 

 crystals, and of scattered grains of quartzose sand. 



The amount of silica disseminated through the rock is less than, I 

 think, is commonly supposed, which is likewise true of the quartzose 

 sand. The granular character of the rock causes it to weather to a 

 harsh sand-like surface, which appears more silicious than it really is. 

 Judging from the analyses made, the silicious ingredient rarely ex- 

 ceeds ten per cent., and occasionally falls below two. 



The chert, of which the quantity, on the whole, is large, is not con 



